Oak and birch were used as precursors to produce the activated carbons (ACs) with well-developed mesoporosity by phosphoric acid-promoted activation in a steam atmosphere. The effect of experimental variables such as the amount of activating agent, the soaking time and the type of wood on the development of porous structure upon heating at 480 degrees C was investigated. The materials were characterized by N-2 adsorption at 77 K, mercury porosimetry and elemental analysis. It was demonstrated that increasing impregnation ratio favors the development of micropores and small mesopores of 2-5 nm, whereas the soaking time promotes the creation of large mesopores, between 10 and 50 nm. Compared to birch, the oak activation using phosphoric acid in the same conditions gives ACs with lower mesopore volume and higher contribution of small mesopores that reflects the differences between both precursors in their biopolymer composition. The presence of steam in the H3PO4 activation process compared to nitrogen facilitates the development of mesoporosity to much higher extent for the birch than that of oak. The ACs prepared in this work show the BET surface area ranging from 800 to 2250 m(2) g(-1), the total pore volume of 0.35-2.04 cm(3) g(-1) with mesopore fraction between 0.06 and 0.68. (C) 2007 Elsevier Ltd. All rights reserved.